Magnetically geared machine for marine generation

ABSTRACT

The present invention provides an electrical machine for use in marine generation comprising a marine turbine, a first rotor mechanically connected to the marine turbine, a second rotor and a stator. The first rotor is configured to transfer torque to the second rotor in a magnetically geared manner, and the second rotor is configured to induce an AC voltage in the stator.

The present invention relates to energy generation, and in particular energy generation in a marine environment.

Using renewable resources, such as tidal or wind power to drive turbines to produce energy is an increasingly desirable process in order to reduce the use of fossil fuels and carbon emissions. Extracting energy from turbines in tidal streams has several advantages over extracting energy from wind turbines. In particular, tidal flows are periodic and predictable and, due to the relative densities of water, a smaller turbine swept area is required for a given power.

Useful power levels for utility scale generation in a tidal flow turbine range from 100 kW to several megawatts (MW) per turbine. The speeds of rotation of the turbine are very low, in the order of 5-15 rpm and are often limited by cavitation effects which occur at high tip speeds at the turbine blade. Therefore, the torques required are extremely large (P=wT) and can be of the order of mega-Newton-meters (MNm).

Traditionally, a mechanical gearbox would be used to increase the rotational speed from the turbine, stepping down torque to allow a more compact generator (i.e operating >1000 rpm) to be employed. However, the use of mechanical gearboxes introduces significant disadvantages in terms of sealing, lubrication, reliability and servicing. It also introduces further losses, which can take up a significant proportion of the available power when working at low loads, thus reducing efficiency further. Lower efficiencies reduce the turbine's electrical output and increase payback time; the time taken by the turbine to pay for itself

A mechanical gearbox typically shares lubrication circuits with the main support bearings in a turbine arrangement. Debris and detritus generated by the wear of the gears in the gear train will therefore travel to the bearing surfaces, often leading to increased wear on the bearing surfaces and hence, premature bearing failure. Any failures and/or servicing that requires the turbine to be retrieved from the water will significantly increase payback time. Also a large volume of transmission oil is required to ensure correct lubrication of the gearbox, which means there is a potential for damaging environmental contamination if a leak should occur. This is a distinct disadvantage in a environmentally friendly energy generation system.

The problems with servicing and replacement following failure are compounded by the fact that likely sites for tidal energy farms are remote and at increasing depth (as these sites have the highest energy densities), leading to any required maintenance or replacement intervention being risky and expensive.

Hence, it is attractive to remove the mechanical gearbox from the power train, but the resulting size of a direct drive generator, connected directly to the blades is prohibitive in terms of hydrodynamic flows.

A key factor in ensuring the successful uptake of marine energy systems is therefore the development of highly reliable, and compact power train systems which have the ability to operate with minimal intervention over extended periods of time.

The present invention seeks to address the above problem through the use of magnetic gearing in the place of a mechanical gearbox. The use of magnetic gearing can provide significant advantages as will become apparent from the following detailed description.

An electrical machine, such as the electrical machine disclosed in WO 2007/125284, could be used in the generation of AC current from the rotation induced by a turbine in a tidal flow. However, disadvantages with hydrodynamic flow and complexities in marinisation exist which preclude such an approach.

The ability of a conventional electrical machine to produce torque is determined by the achievable shear force density in the airgap (a function of magnetic field flux density and the stator currents) which is limited by material properties (i.e. maximum flux densities achieved by permanent magnets and/or due to the saturation levels in the steels in the magnetic circuit) and thermal limits due to losses induced due to non-perfect winding conductors. In order to achieve a large torque in a conventional electrical machine, the rotor surface area must be large. Large rotor diameters are more beneficial than long rotors as diameter increases both the area and the torque arm effect, and long lengths lead to difficulties due to the required separation between bearing supports and hence, rotor dynamics. However, a large machine diameter would detrimentally affect hydrodynamic flow and would reduce efficiency of energy extraction. Given that more power can be extracted from a smaller turbine, the size of the nacelle housing the generator becomes more significant than in wind turbines; a smaller nacelle is desirable. Hence there is a need to minimise nacelle size whilst achieving suitable power generation in a marine environment. A magnetically geared electrical machine enables a smaller generator for a given torque requirement to be realised. Whilst, so far, tidal flow generation has been focussed upon, the present invention is equally applicable to generation for tidal barrages and fences.

The present invention aims to solve the aforementioned problems and provides an electrical machine for use in marine generation comprising, a marine turbine; a first rotor mechanically connected to the marine turbine; a second rotor; a stator; wherein the first rotor is configured to transfer torque to the second rotor in a magnetically geared manner, and wherein the second rotor is configured to induce an AC voltage in the stator windings.

The electrical machine may further comprise an array of pole pieces; a first plurality of magnets and a second plurality of magnets.

Preferably the pole pieces are provided on the first rotor, the first plurality of magnets is provided on the second rotor and the second plurality of magnets is provided on the stator.

Preferably the first rotor, the second rotor and the stator are arranged concentrically around a shaft of the marine turbine. Alternatively, the first rotor, the second rotor and the stator are arranged axially along a shaft of the marine turbine. The pole number of the first plurality of magnets is different from the pole number of the second plurality of magnets and the gearing ratio between the first rotor and the second rotor is based on the ratio between the pole number of the first plurality of magnets and the pole number of the second plurality of magnets or the ratio between the number of pole pieces and the number of pole pairs of the first plurality of magnets. The gearing ratio may be equal to (P₂/P₁)+1 where P₁ is the number of pole pairs of the first plurality of magnets and P₂ is the number of pole pairs of the second plurality of magnets. Alternatively, the gearing ratio may be equal to N_(pp)/P₁ where N_(pp) is the number of pole pieces and P₁ is the number of pole pairs of the first plurality of magnets.

The electrical machine may further comprise a winding, wherein the winding is arranged to interact with the fundamental space harmonic of the magnetic field created by the first plurality of magnets, thereby inducing a voltage in the windings. The winding may be disposed on the stator.

Due to magnetic gearing, the torque that needs to be reacted due to interaction of stator currents and magnets on the first rotor is reduced. Therefore, the active airgap area can be reduced and/or the level of current can be reduced, which has efficiency benefits. Hence, torque is geared down between the first rotor and the second rotor, reducing the active air gap and/or reducing stator mmf required between the second rotor and the stator.

Preferably the second rotor, which is usually the higher speed rotor, is sealed within an enclosure formed by the first rotor.

Preferably a gap is provided between the stator and the first rotor which is open to the incursion of fluid.

The magnetic poles on the first and second plurality of magnets may be produced by permanent magnets or electromagnets.

The first rotor may comprise ferromagnetic pole pieces encapsulated within a frame comprising a non-magnetic material.

The frame forms an enclosure around the second rotor. The frame can comprise end pieces connected to a shaft of the turbine which form an enclosure around the second rotor.

Another aspect of the invention provides an arrangement comprising: one or more electrical machines according to the present invention; one or more passive or active rectifiers configured to convert the induced AC voltage to DC; and one or more inverters to convert the DC voltage to AC voltage having a predetermined magnitude and frequency.

Preferably in the above arrangement the inverter is connected to a power grid, and the predetermined magnitude and frequency are the magnitude and frequency of the power grid. The number of rectifiers can correspond to the number of electrical machines, and each electrical machine can be provided with a local rectifier, wherein each rectifier is preferably a passive rectifier.

The one or more inverters may comprise a single inverter.

One or more embodiments of the present invention are described below with reference to the accompanying drawings, in which:

FIG. 1 a shows a cross section of a magnetic gear.

FIG. 1 b shows an alternative cross section of the magnetic gear of figure la.

FIG. 2 a shows a cross section of a magnetically geared generator driven by turbine blades in accordance with the present invention.

FIG. 2 b shows a cross section of the magnetically geared generator of FIG. 2 a

FIG. 3 shows a cross section of a magnetically geared generator in use in a marine environment.

FIG. 4 shows an alternative embodiment of a magnetically geared generator.

FIG. 5 shows a number of cross-sections of the magnetically geared generator of FIG. 4.

FIGS. 1 a and 1 b show a magnetic gear 1 employing a single level of gearing.

However, the invention encompasses multiple levels of gearing too. The gear 1 comprises a first, outer rotor 2 having a support 21 bearing a first plurality of permanent magnets 23 to produce a spatially varying magnetic field. The first, outer rotor 2 may be connected to a drive shaft 50. The gear 1 further comprises a second, inner, rotor 3 disposed concentrically within the outer rotor 2 and consisting of a support 31 having a second plurality of permanent magnets 33 disposed thereon to produce a spatially varying magnetic field. The first and second pole numbers of permanent magnets are different. Accordingly, without any other interference/modulating means there will be little or no useful magnetic coupling or interaction between the permanent magnets 23 and 33 such that rotation of one rotor would not cause rotation of the other rotor. The second rotor 3 may be connected mechanically to a generator.

Each rotor is arranged to rotate about the same axis, and each is mounted using suitable bearings, which are not shown.

A static stator 4, comprising ferromagnetic pole pieces 43 acts to modulate the magnetic field of the first rotor 2 and the second rotor 3. The pole pieces 43 modulate the magnetic field to produce a spatial harmonic that has the same pole number as the second plurality of magnets 33, which enables magnetic coupling. If the first plurality of magnets 23 rotates, this spatial generated harmonic travels at a different speed to the rotor producing it. The second plurality of magnets 33 is synchronised with this higher-order spatial harmonic and so the second rotor 3 rotates asynchronously with the first rotor 2, thereby gearing speed. Gearing is achieved through selection of appropriate numbers of permanent magnets. The ratio of the pole numbers of permanent magnets sets the gearing ratio. The first plurality of magnets 21 of the first rotor 2 are thereby coupled to the second plurality of permanent magnets 33 disposed on the second rotor 3 so that torque is transferred from the first rotor 2 to the second rotor 3 in a magnetically geared way across an airgap. The speed of the second rotor 3 may therefore be geared up or down in comparison to the speed of the first rotor 2.

In this embodiment the static stator 4 is disposed between first rotor 2 and the second rotor 3 such that the pole pieces are aligned between the permanent magnets 23, 33 disposed on the first and second rotor. The first rotor 2, the second rotor 3 and the static stator 4, are arranged concentrically. A circumferential airgap exists between the first rotor and the stator and between the stator and the second rotor. However, the stator can alternatively be placed outside the first rotor 2 or be disposed concentrically within the second rotor 3.

In the embodiment shown, the stator 4 consists of a support 41 which acts as a housing and forms an enclosure in which the second rotor 4 is contained, preferably within a dry volume 6. The ferromagnetic pole pieces 43 are encapsulated within the support 41, and thus protected from external influences.

As will be appreciated from the above, a gearbox using the principles of magnetic gearing exploits the interaction between a first set of magnets and a second set of magnets to transmit rotation forces or torques across an airgap. As the first rotor 2 comprising the first plurality of permanent magnets rotates, the second rotor 3 comprising the second plurality of permanent magnets 130 is caused to rotate at a higher speed reducing torque because the stator 4 comprising the pole pieces 43 is static.

A magnetic gear, such as the one shown in FIG. 1 a, does not have contacting surfaces, or engaging teeth (avoiding the generation of detritus caused by such mechanical features) and does not require transmission oil. There is no lubrication to be periodically replaced. The bearing lubrication circuits can be highly optimised and readily marinised so that such a gearbox is highly reliable and highly suitable for use in a marine environment.

As torque is transmitted across an airgap, a membrane or seal wall can be disposed between the first rotor 2 and the second rotor 3 and the second rotor 3 can be enclosed within a housing, so as to enable it to be isolated from the marine environment, thereby avoiding the requirement for a dynamic rotating seal to isolate the dry running components from a wet environment, which is required in current mechanisms. The use of such a magnetic gearbox in a marine power generation application therefore overcomes problems with current solutions. Although in the foregoing description the pole-pieces are described as static, geared rotation can also be achieved by providing an outer magnet array fixed on the stator, and providing the pole-pieces on or as a rotor.

In a preferred embodiment of the invention, as shown in FIG. 2, a magnetic gear is integrated within a generator. Referring to FIG. 2, a magnetically geared generator comprises an array of pole pieces 120, a first plurality of magnets 130 and a second plurality of magnets 140.

The pole pieces 120 form a rotatable array of pole pieces which comprises a first, low-speed, rotor 2. The low-speed rotor 2 is mechanically connected to a turbine 6 via a shaft 50. The pole pieces 120 are arranged in a radial spoke pattern. Preferably, the pole pieces 120 are housed within a non-magnetic matrix 121 with end plates 123 connected to the shaft 50 to carry the torque from the shaft 50. A magnetic pole-piece support is disclosed in the applicant's co-pending application WO 2009/138728 which is incorporated herein by reference.

A second, high-speed, rotor 3 is arranged around the output shaft 50 and includes the first plurality of permanent magnets 130. The high speed rotor 3 is not mechanically connected to the first rotor 2 and is free to rotate in relation to the main output shaft 50 of turbine 6 through the use of suitable bearings.

A static stator 4 can be positioned outwardly of the first rotor. The stator includes the second plurality of permanent magnets 140 and is associated with a plurality of multi-phase windings 142.

In a preferred embodiment, as shown in FIGS. 2 and 3, the generator has an external stator 4 in a radial field configuration in which the first rotor 2, second rotor 3 and stator 4 are arranged concentrically about the main shaft 50 and define two circumferential airgaps. Alternatively, the stator 4 is internal to the second rotor 3 with an intermediate first rotor 2 comprising the pole-piece 120 array.

In use, the turbine 6 transfers torque to the low-speed rotor 2 via the shaft 50. The pole pieces 120 on the first rotor 2 modulate the magnetic field between the first plurality of permanent magnets 130 and the second plurality of permanent magnets 140. The magnetic fields of the static magnets on the stator 4 and second rotor 3 can then interact such that the low speed rotation of the first rotor 2 is converted to a high speed of rotation on the second rotor 3. Coupling between the first plurality of magnets 130 and the second plurality of permanent magnets 140 is realised through the rotatable pole pieces 120. Although the pole pieces 120 have no effect on the interaction between the windings 142 on the stator 4 and the first plurality of permanent magnets 130, the presence of the pole pieces 120 produces an asynchronous harmonic with the same number of poles as the static second set of permanent magnets 140. The first plurality of magnets 130 and the second plurality of magnets 140 are thus coupled. As the first rotor 2 comprising the array of pole pieces 120 rotates, the second rotor 3 rotates at a geared up speed because the second plurality of permanent magnets 140 is static. The ratio between the pole number of the first plurality of magnets and the pole number of the second plurality of magnets sets the gearing ratio between the first rotor and the second rotor. As the second plurality of magnets is static, the gear ratio is equal to (P₂/P₁)+1 where P₁ is the number of pole pairs of the first plurality of magnets and P₂ is the number of pole pairs of the of the second plurality of magnets. Alternatively, the gear ratio can be written as N_(pp)/P₁ where N_(pp) is the number of pole pieces and P₁ is as above. The number of pole pairs of each plurality of permanent magnets is half the pole number of the plurality of magnets. Gearing is as required for the specific implementation of the invention.

The first plurality of permanent magnets 130 disposed on the second rotor 3 is arranged so that the first or fundamental harmonic couples with or forms magnetic circuits with the multi-phase winding 142 as the second rotor 3 rotates. As a result, AC voltages and currents (when connected to a load) are induced in the windings 142 disposed on the stator. In a preferred embodiment, the multi-phase windings 142 associated with the stator may be 3-phase windings. Alternatively, any multi-phase winding such as a 5-phase winding may be used. In a further alternative embodiment, a winding 142 may comprise a number of independent coils or circuits to enable redundancy or fault tolerance, known as duplex or triple windings. In this embodiment, in the event of one winding becoming open-circuit or short circuited the machine can still function.

One skilled in the art understands how to select and design the pole pieces given the first 130 and second set of permanent magnets 140 to achieve the necessary magnetic circuit or coupling such that gearing between the first rotor 2 and the second rotor 3 as can be appreciated from, for example, K Atallah, D. Howe, “A novel high-performance magnetic gear”, IEEE Transactions on Magnetics, Vol. 37, No. 4, pp. 2844-2846, 2001 and K Atallah, S. D. Claverly, D. Howe, “Design, analysis and realisation of a high performance magnetic gear”, IEE Proceedings-Electric Power Applications Vol. 151, pp. 135-143, 2004, which are incorporated herein by reference in their entirety.

In gearing up speed, the torque capacity required of the electrical generator (due to interaction of the stator winding currents and excitation field created by the second rotor) is geared down and therefore the generator only needs to handle a low torque, and the active area of the airgap can be reduced thereby minimising the size of the generator. Additionally, due to the gearing down of the torque, the stator windings are required to cope with a lower applied magnetomotive force(mmf). The mmf is the product of current and turns and so less turns are required on the winding, reducing the size of the winding, and as a result, the slots in which the windings are housed, reducing the size of the stator. Further, the coils operate at a much lower current density and hence copper losses are reduced and efficiency increased.

The resulting machine is therefore significantly smaller than a conventional direct drive machine allowing a compact nacelle which will not adversely influence the hydrodynamic flows and provides higher hydro-dynamic efficiency. The resulting machine is approximately 30 to 50% of the volume of a direct drive machine and comparable in size with a combined generator and gearbox. In a machine for use in a marine environment, the diameter of the machine is restricted due to the stresses on the high speed rotor. Due to this restriction the present invention is competitive with a direct drive machine of a similar diameter for use in a marine environment and has a comparative axial length of approximately 30-50%.

In a permanent magnet excited system, the peak torque capability of the magnetic gear element is determined by the magnetic fields set by the magnetic material, magnet geometry, airgaps and pole piece thicknesses. The peak torque at which the gear slips can be accurately predicted and variation of magnetic performance with, for example, temperature, can be accounted for (although manufacturing and material composition tolerances affect this). The gear torque transmission capability can be designed such that if a pre-determined torque is exceeded, the rotors will harmlessly slip relative to each other which can be used to protect the transmission system and removes the requirement for safety clutches. However, it is likely that the gear element would be rated such that it never slips with a torque overhead (˜30%) applied. The level of overhead is the level of torque over the normal operating torque and is determined by considering the capability (speed) of the machine controller, the pitch on the blades and the peak torques due to, for example, waves, swell etc. However, the gear may be designed to slip on the very extreme loads that occur very infrequently.

The magnets of the rotors are carefully segmented and pole pieces 120 may be laminated, reducing losses in the magnetic gear elements caused by hysteresis and eddy current loss in the pole pieces and eddy currents in the magnets. For a radial machine, the pole-pieces 120 are laminated in the axial direction. Although it is necessary to laminate the pole pieces 120 to impede the flow of eddy currents, this can reduce the mechanical strength of the pole pieces which must bear the rotational torques and magnetic forces. Thus the poles are carried within a composite structure (as the space between the pole pieces needs to be non-magnetic and non-conducting) as discussed, for example, in WO 2009/138728 and WO 2009/138725. The losses in the magnetic gear elements are also relatively constant at a given speed. As the currents, and hence losses, in the machine element are minimised the requirement for cooling is reduced, which also increases utility in the marine environment.

As the internal gearing is derived from a non-contacting magnetic gear element instead of a mechanical gearbox, it has the advantage of no gear wear and hence lubricant free operation, reducing maintenance requirements and the number of interventions, which is highly desirable.

FIG. 3 shows the magnetically geared generator of the present invention in use in a marine environment. The generator comprises a pole piece array 120, a first plurality of permanent magnets 130 and a second plurality of permanent magnets 140 as already noted. The pole piece array 120 is disposed on a first rotor 2 which can be placed in contact with water and which is mounted on the turbine shaft. The pole pieces 120 are, preferably, over moulded (disposed within the material of the first rotor) to provide a sealing surface and to reduce the potential for corrosion. Suitable marine protection is provided to the outer surface of the first rotor 2 to prevent biofouling and/or corrosion of the outer surface. The end plates 123 of the first rotor form, with the first rotor, a frame or housing in which the first plurality of magnets 130 is enclosed. It is to be noted that the housing may be non-magnetic in its entirety or may be a hybrid structure of magnetic and non-magnetic materials. For example, to reduce cost, the housing may be a hybrid structure of structural steel and non-magnetic material (such as carbon-fibre or other suitable non-magnetic material). In any form of hybrid structure, the housing is comprised of non-magnetic material in the airgap region between rotors to facilitate modulation. The first plurality of permanent magnets 130 is disposed on a second rotor 3 which is free to rotate relative to the first rotor 2. The first rotor 2 and the second rotor 3 are arranged concentrically about the main turbine shaft. The structure of the first rotor 2 acts as a seal or barrier to fully enclose the second rotor 3. As the second rotor operates in a dry environment, there is no viscous drag on the higher speed component from the water in which the apparatus operates, and the high speed bearings do not have to be designed to withstand the harsh marine environment.

The second plurality of permanent magnets 140 is disposed on a static stator 4 which is disposed outwardly of, and arranged concentrically with, the first rotor 2. The stator 4 is open to the environment, thus operating in use in a flooded condition. The stator 4 is static and there is therefore no requirement for dynamic sealing. The flooding of the stator 4 and the outer surface of the low-speed rotor 2 provides significant cooling of the rotor 2 and the windings 142 and removes the requirement of dynamic seals. In an alternative embodiment, however, the stator 4 may be encapsulated as shown in FIG. 2.

In use the first rotor 2 transfers torque induced by a turbine across an airgap to the second rotor 3 in a geared manner as described above. The first set of permanent magnets 130 and the second set of permanent magnets 140 are coupled by way of the pole pieces 120. As the first rotor 2 comprising the pole pieces 120 rotates, the second rotor 3 comprising the first plurality of permanent magnets 130 is caused to rotate at a higher speed because the second set of permanent magnets 140 is static. The multi-phase winding 142 associated with the stator 4 interacts with the fundamental space harmonic of the magnetic field created by the first set of permanent magnets 130 on the second rotor 3. The second rotor 3 thereby induces an AC voltage on the windings associated with the stator. Thus AC voltage may be captured for use as will be described later.

In an alternative embodiment, the machine can be turned effectively inside out and have an internal stator having a plurality of permanent magnets, arranged concentrically with the first and second rotor, wherein the second rotor, which is magnetically geared to a first rotor so that its rotational speed is higher than the first rotor is disposed externally to the stator. A pole piece array is disposed in the airgap between the first rotor and the stator to couple the magnetic fields.

As stated above, marine protection systems may be used to prevent biofouling or corrosion. Magnets and steel components may, by way of example, be protected by electroplating, nickel or aluminium coatings. The stator, windings and outer magnets may be encapsulated in an appropriate epoxy. The outer surfaces of the pole pieces and steel etc. may have anti-corrosion ceramic coatings, e.g. products such as Ceramax or Ultraglide. Biocidal fouling control coatings or antifouling coatings such as Copper filled epoxy coatings (coppercoat) may also be used. Structural steel components may also need cathodic protection in the submerged zone achieved using sacrificial anodes.

FIG. 4 shows an alternative embodiment of a magnetically geared generator in which the rotors are arranged as a series of discs (221,231,241) on a common axis along a turbine shaft 50. An axial field is produced with airgaps along the axis and between the discs. In this embodiment, a static stator 241 accommodates a winding 242 and has an array of permanent magnets 240 mounted proximate to the airgap which separates the stator from a first rotor 221 having an array of pole pieces 220 disposed thereon, adjacently facing the permanent magnetic array 240 of the stator 241. The first rotor 221 is mounted on the turbine shaft 50 such that it rotates with the turbine shaft 50. Preferably the pole pieces 220 are arranged in a radial spoke pattern. A non-magnetic support structure 223 may be placed between these pole-pieces to prevent deflection of the pole-pieces 220 due to unbalanced magnetic pull and the torsion due to the load torque. This structure 223 may form the rotor and also a frame or enclosure to house a second rotor as will be described further below. However, the support structure may form only a part of the rotor and frame/enclosure; the region adjacent the airgap between rotors in order to facilitate modulation. In such a case the remainder of the rotor and frame/enclosure may be formed of a magnetic material. For example, to reduce cost, the rotor/frame/enclosure may be a hybrid structure of structural steel and non-magnetic material (such as carbon fibre, glass fibre or other suitable non-magnetic materials) with the non-magnetic material forming the support structure 223.

A second airgap separates the first rotor 221 from a second rotor 231. The second rotor 231 has an array of permanent magnets 230 disposed thereon and is positioned adjacent to the first rotor 221 and mounted on the turbine shaft 50 by a bearing system such that the second rotor 231 can rotate freely about the turbine shaft 50. Alternatively, the first rotor 221 surrounds the turbine shaft 50 and extends along the shaft externally and, possibly, internally to a frame or enclosure arranged around the second rotor, and formed by the first rotor 221. In the embodiment in which the first rotor 221 extends along the shaft 50 internally to the frame or enclosure formed by the first rotor 221 as shown in FIG. 4, the second rotor 231 is mounted on the first rotor 221, such that the second rotor 231 can rotate relative to the first rotor 221 and thus the turbine shaft 50. The second rotor 231 preferably comprises a ferromagnetic disc including an array of permanent magnet poles 230. The ferromagnetic disc provides support and a back-iron flux return path. As can be seen in FIG. 4, the first rotor 221 houses the second rotor in a frame or enclosure so that the second rotor 231 acts in a dry environment, as discussed above in relation to the concentric arrangement.

FIG. 5 shows a number of sections through the electrical machine of FIG. 4 at A-A′, B-B′, C-C; and D-D′. As best seen through cross-section A-A′, the permanent magnet array mounted second rotor is formed of alternating magnetic poles. Cross-section B-B′ illustrates the first rotor carrying magnetic pole pieces and cross-section C-C′ illustrates the static permanent magnet array mounted on the inner surface of the stator, which is shown in cross-section D-D′.

Although the above embodiments have concentrated on permanent magnet excited motors, the excitation on the high speed-rotor may be provided by a field coil or an electromagnet. Magnet to magnet coupling offers higher torque densities than magnet to coil coupling. A wound field electrical machine is preferably employed in a radial field form with a high-speed rotor and an external stator for simplicity and ease of manufacture. The static pole array can still be produced by an array of permanent magnets rather than electromagnets although using static wound electromagnet arrays is also an option. If static wound electromagnetic arrays are used, a controllable field source for the generator is obtainable, improving the control over the system.

Although varying the current in the field winding to vary the field for the motor also alters one of the field sources for the magnetic gear element, thereby reducing the peak torque capability, this can be an advantage as it reduces iron losses in the system by reducing field levels when torque requirements are low. In this embodiment, however, additional copper losses are present due to the field coil. It also allows a short term, transient increase in the torque capability by increasing the current in the field windings.

Ultimately, the output from the magnetically geared generator must be synchronised to the power grid be that a national grid and/or a local grid in an islanded power system potentially with other power generators feeding this grid system. By synchronisation, the output must have a matching AC voltage and frequency to the grid. As the generator is a variable speed generator, it cannot be connected directly to the grid; it requires interface power electronics. The electronics condition the power developed by the generator such that it is compatible with the grid supply and also aid with fault handling and compliance issues such as low voltage ride through.

The AC voltages generated by the generator can be put through a passive or active rectifier to convert to an intermediate DC voltage. Transient energy storage, such as capacitors, may also be used at this stage. This stage is followed by an active inverter which converts the DC back to a fixed frequency AC signal compatible with the power grid. Alternatively, an AC-AC converter or Matrix converter could be employed, removing the requirement for an intermediate energy store/filter.

Given accessibility issues and cost of interventions and potential opportunity for device failure, robust power electronics are essential. Therefore, the magnetically geared generator may be connected to a simple and robust passive rectifier, and thus locally transmit power as DC. A series of turbines (or “farm”) could be connected to a smaller number of inverters. The inverters may be sub-sea, on a floating buoy or land-based inverters, which convert DC power to AC power synchronised to the power grid. For turbine farms in remote locations the umbilical cable connecting the group of turbines may be DC given the long distances for power to be transmitted. This reduces AC losses in the cabling. An on-shore inverter would then be employed to connect to the grid.

The generator for such a system must be designed to ensure good regulation of voltage by maintaining voltage at the generator terminals as increasing current is output. This requires that the winding impedance, both reactance and resistance is carefully designed to reduce the voltage drop across these components. This is achieved through selection of slot/tooth geometry, working airgaps and winding arrangements. The magnetically geared machine tends to have excellent regulation capability as it has low inductance/reactance, a large effective airgap presented to the coil generated fluxes, and the machine would typically have a power factor >0.9. In some cases it may be necessary to tailor the reactance of the machine (e.g. by altering the slot geometry to alter slot leakage) in order to limit fault currents. However, in optimising for voltage regulation, there would be compromises in other performance parameters such as torque density, volume of copper and stator mass/cost etc.

It will of course be understood that the present invention has been described above purely by way of example and modifications of detail can be made within the scope of the invention. 

1. An electrical machine for use in marine generation comprising: a marine turbine; a first rotor mechanically connected to the marine turbine; a second rotor; a stator; wherein the first rotor is configured to transfer torque to the second rotor in a magnetically geared manner, and wherein the second rotor is configured to induce an AC voltage in the stator.
 2. The electrical machine of claim 1, further comprising an array of pole pieces; a first plurality of magnets; and a second plurality of magnets.
 3. The electrical machine of claim 2, wherein the pole pieces are provided on the first rotor, the first plurality of magnets is provided on the second rotor and the second plurality of magnets is provided on the stator.
 4. The electrical machine of claim 1, wherein the first rotor, the second rotor and the stator are arranged concentrically around a shaft of the marine turbine.
 5. The electrical machine of claim 1, wherein the first rotor, the second rotor and the stator are arranged axially along a shaft of the marine turbine.
 6. The electrical machine of claim 1, wherein the pole number of the first plurality of magnets is different from the pole number of the second plurality of magnets and the gearing ratio between the first rotor and the second rotor is based on the ratio between the pole number of the first plurality of magnets and the pole number of the second plurality of magnets or the ratio between the number of pole pieces and the number of pole pairs of the first plurality of magnets.
 7. The electrical machine of claim 3, wherein the gearing ratio is equal to (P2/P1)+1 where P1 is the number of pole pairs of the first plurality of magnets and P2 is the number of pole pairs of the second plurality of magnets.
 8. The electrical machine of claim 3, wherein the gearing ratio is equal to Npp/P1 where Npp is the number of pole pieces and P1 is the number of pole pairs of the first plurality of magnets.
 9. The electrical machine of claim 1, further comprising a winding, wherein the winding is arranged to interact with the fundamental space harmonic of the magnetic field created by the first plurality of magnets, thereby inducing a voltage in the winding.
 10. The electrical machine of claim 9, wherein the winding is disposed on the stator.
 11. The electrical machine of claim 1, wherein torque is geared down between the first rotor and the second rotor, reducing the required active air gap area between the second rotor and the stator.
 12. The electrical machine of claim 1, wherein torque is geared down between the first rotor and the second rotor reducing the required stator mmf.
 13. The electrical machine of claim 1, wherein the second rotor is sealed within an enclosure formed by the first rotor.
 14. The electrical machine of claim 1, wherein a gap is provided between the stator and the first rotor which is open to the incursion of fluid.
 15. The electrical machine of claim 1, wherein the pole pieces and the first and second plurality of magnets are permanent magnets or electromagnets.
 16. The electrical machine of claim 1, wherein the first rotor comprises ferromagnetic pole pieces encapsulated within a non-magnetic portion of a frame.
 17. The electrical machine of claim 16, wherein the frame forms an enclosure around the second rotor.
 18. The electrical machine of claim 17, wherein the frame comprises end pieces connected to a shaft of the turbine which form an enclosure around the second rotor
 19. An arrangement comprising: one or more electrical machines according to claim 1; one or more passive or active rectifiers configured to convert the induced AC voltage to DC; and one or more inverters to convert the DC voltage to AC voltage having a predetermined magnitude and frequency.
 20. An arrangement as claimed in claim 19, wherein the inverter is connected to a power grid, and the predetermined magnitude and frequency are the magnitude and frequency of the power grid.
 21. An arrangement as claimed in claim 19, wherein the number of rectifiers corresponds to the number of electrical machines, and each electrical machine is provided with a local rectifier.
 22. An arrangement as claimed in claim 21, wherein each rectifier is a passive rectifier.
 23. An arrangement as claimed in claim 19, wherein the one or more inverters comprises a single inverter.
 24. (canceled)
 25. (canceled) 